1. Field of the Invention
The present invention relates to a digital camera that stores in memory a subject as image data that are electronically compressed, and it also relates to a storage medium that stores an image signal processing program. Furthermore, the present invention relates to a carrier wave that is encoded to transmit a control program for white balance adjustment on image data. It also relates to an electronic camera that allows selection to be made between recording of irreversible image data and recording of raw data.
2. Description of the Related Art
Electronic still cameras in the known art include the type provided with a viewfinder device to which a subject image having passed through a taking lens is guided by a quick return mirror, an image-capturing device such as a CCD that is provided at a rearward of the quick return mirror to capture an image of the subject image and output image data, an image processing circuit that performs image processing such as white balance and gamma correction on the image data output by the image-capturing device, a compression circuit that compresses the data which have undergone image processing through a method such as JPEG and stores the data in a storage medium such as a flash memory and a monitor that displays the data having undergone the image processing. In the image processing circuit, parameters such as the R gain and the B gain for white balance adjustment or the gradation curve for gamma correction are calculated based upon a preset algorithm. In addition, the image data are converted to 16×8 sets of brightness data Y and 8×8 sets of Cr and Cb color difference data for JPEG compression.
The image-capturing device in such an electronic still camera in the prior art structured as described above presents the following problems.
(1) Both the image pre-treatment such as white balance or gamma correction and the image post-treatment, in which the data that have undergone the image pre-treatment are formatted for the JPEG compression, are performed in units of individual lines in correspondence to the read performed at the CCD. Because of this, in a high image quality electronic still camera with the number of pixels at the CCD exceeding two million, the capacity of the line buffer memory employed for pipeline operation and the like, is bound to be very large, resulting in the camera becoming expensive. This problem may be explained as follows.
When performing signal processing on the output from a solid image-capturing element, N×M sets of image data corresponding to one screen output by the image-capturing element are output in point sequence in units of individual lines. Thus, when performing signal processing including pixel interpolation processing and filtering processing, a line buffer memory supporting four lines, for instance, is required if the filtering processing is to be performed in sets of 5×5. In other words, the processing can be performed only when image data corresponding to four lines have been accumulated in the memory. Such a line buffer memory supporting four lines is required for each of the various types of processing such as filtering processing and interpolation processing.
If a line buffer memory that supports four lines is provided at a 1-chip processing IC for each of the various types of processing required, such as the filtering processing and the interpolation processing described above, the ratio of the area occupied by the memory increases, which leads to an increase in the number of gates at the 1-chip processing IC, resulting in higher cost. In particular, in a high resolution type image-capturing element having more than two million pixels with a large number of pixels per line, the cost will be especially high. In addition, if the line buffer memory is provided outside the 1-chip processing IC, twenty 10-bit input/output pins, for instance, will be required. This means that 20 input/output pins will be necessary for each line buffer memory to result in an increase in the package size of the 1-chip processing IC.
(2) In the image-capturing device in an electronic still camera in the prior art, the interpolation processing for an (R−G) signal and a (B−G) signal, matrix processing through which a Y signal, a Cr signal and a Cb signal are generated using the (R−G) signal, the (B−G) signal and the G signal and LPF processing through which low frequency signals are extracted from the Y signal, the Cr signal and the Cb signal is performed in time sequence to format the data for JPEG compression and to suppress false colors and color moire from occurring. As a result, particularly in the case of a high image quality electronic still camera with the number of pixels at the CCD exceeding two million, the length of time of processing is bound to be large, resulting in poor operability.
(3) In the image-capturing device in an electronic still camera in the prior art, a single primary color type CCD, two CCDs (one for G and the other for R/B) or three CCDs (one each for R, G and B) are employed. When using a single CCD, since an RGB color filter is provided at the front surface of each pixel at the CCD, an R signal, a G signal or a B signal is missing from a given pixel. Thus, interpolation is performed for pixels without a G signal component by using the G signals of pixels that have been actually obtained to generate G signals for all the pixels, and interpolation in regard to the (R−G) signal and the (B−G) signals is likewise performed. The same principle applies when using two CCDs, as well.
However, depending upon the nature of the image that has been captured or the characteristics of the low pass filter employed, false colors or moire may occur after the interpolation processing, which results in a great degree of degradation in the image quality. While the Cr signal and the Cb signal among the Y signal, the Cr signal and the Cb signal described above created using the R, G and B signals are processed through the low pass filter to suppress false colors or moire in the prior art, this means does not achieve satisfactory results in a high image quality electronic still camera having more than two million pixels at the CCD.
(4) Since the white balance adjustment is achieved using predetermined white balance adjustment coefficients in the image-capturing device in an electronic still camera in the prior art, there is the likelihood of a color-cast image being generated if the white balance adjustment coefficients are set erroneously. This problem tends to occur more readily in a high image quality electronic still camera with the number of pixels at the CCD exceeding two million.
Electronic cameras in which selection between the two different data formats described below can be performed when recording image data obtained through image-capturing have been known in the prior art.
(1) Irreversible compressed data obtained through JPEG or the like that have undergone a sequence of image processing
(2) Raw data output by the image-capturing device
The first type of data, i.e., irreversible compressed data are advantageous in that since the code volume is relatively small, a large number of images can be stored in an external recording medium such as a memory card. In addition, they are recorded in a general-purpose format which allows data decoded by using a common image viewing software program or the like to be printed or displayed directly.
The second type of data, i.e., raw data are image data faithful to the output signal from the image-capturing device. A data recording format of raw data facilitates external processing. Since raw data which, undergo very little irreversible gradation conversion or data compression, contain a large volume of information such as the number of quantization bits, they have a wide dynamic range as image information. Thus, they provide an advantage in that they can be processed in an ideal manner without the tendency to lose fine gradation components. For this reason, highly advanced data processing and a higher quality are required in this type of raw data. The data format of raw data is particularly suited for printing and design applications.
Normally, an electronic camera requires a greater length of time for image processing compared to cameras using a silver halide film. In order to achieve a degree of operability in an electronic camera comparable to that of cameras using a silver halide film, it is crucial to minimize the length of time required for image processing. However, in an electronic camera in the prior art, a raw data read/write operation performed via an image memory is always necessary. This tends to lead to a delay occurring in signal processing performed on irreversible compressed data by a length of time corresponding to the length of time spent on the raw data read/write.
In addition, processing circuits that perform relatively complex processing, a prime example of which is pixel value matrix operation, are concentrated in a processing unit at a stage preceding the stage for gamma control operation. Since raw data with a large number of quantization bits are handled in this state at these processing circuits, the circuit structures of the processing circuits tend to be complex and there is also a problem of a greater length of time required for signal processing.